Aqueous polyurethane dispersions (PUDs) are well known and have mainly been developed due to the evolution of legislation towards reducing the VOC (volatile organic concentration) and the creation of environmentally friendly products. Water-based or waterborne PUDs have gained increasing importance in a range of applications, due in large part to properties such as adhesion to a range of substrates, resistance to chemicals, solvents and water, abrasion resistance and flexibility. Water-based PUD show very good mechanical and chemical properties and match the regulatory restrictions for low volatile organic compound (VOC) containing raw paints.
Some efforts have also been directed to the development of substantially solvent-free aqueous polyurethane dispersions such as notably described in U.S. Patent Application Publication No. 2011/0306724 A1. Said solvent-free aqueous polyurethane dispersions are characterized by substantially no emission of volatile organic materials.
Aqueous polyurethane dispersions (PUDs) are thus used in the production of a variety of useful polyurethane products for example adhesives, sealants and coatings for various substrates including textile fabrics, plastic, wood, glass fibers, metals and the like. Chemical resistance, abrasion resistance, toughness, tensile strength, elasticity and durability are among the many desirable properties of these coatings. Their main advantages are the versatility of polyurethane (PU) systems and their effectiveness in producing ‘tailor-made’ coatings for a wide range of demanding applications. This results from the wide diversity of raw-materials which can be combined in different ways during the synthesis.
Several processes have been developed for the synthesis of PUD. All of these have a common first step, in which a medium molecular weight polymer (i.e. prepolymer) is formed by the reaction of suitable diols or polyols, in particular polyether, polyester or polycarbonate polyols with a molar excess of diisocyanates or polyisocyanates. Said prepolymer needs to be stabilized in order to allow its dispersion in water. There are two ways of stabilization, the first one consists of polymers stabilized by external emulsifiers, and the second one achieves stabilization by including hydrophilic centers in the polymer. Such hydrophilic centers may be one of three types: non-ionic, cationic and anionic groups. These hydrophilic groups fulfil the function as internal emulsifiers and make it possible to produce stable water-based emulsions. Thus, water-based PUDs are classified into anionic, cationic and nonionic systems.
U.S. Pat. No. 4,237,264 to Noll et al. and U.S. Pat. No. 4,238,378 to Markusch et al. notably describe that for coating applications, the polyurethane polymers can be stably dispersed in water by incorporating said hydrophilic groups or potentially hydrophilic groups into their backbone.
The industrial production of PUDs is nowadays a well-established technology. There are two main synthetic routes to produce PUDs: the acetone process and the pre-polymer process.
Jansson R. C. described in TECHNICAL INFORMATION. LEAFLET NO 0127. January 2004. Pages 1 (11) to 11 (11)) that the pre-polymer mixing process is a versatile and straightforward process for the preparations for PUDs, thereby permitting a great variation in PU formulations. This being said, the selection and the relative amounts of the polyurethane (PU) constitutional components (i.e. isocyanate, polyol component, chain extenders, emulsifiers, in particular internal emulsifiers, organic solvents, water, etc.), in the PUD formulation are thereby important for the targeted coating properties. This is also described by Clausson A. et al. in Paint and Coatings Industry, January 2007, Vol. 23, Issue 1, page 30.
N-Methylpyrrolidone (NMP) has frequently been used as the organic solvent in the preparation of PUDs. However, NMP is now known to cause reproductive toxicity (it is considered as being reprotoxic) and is being labeled in the EU as “reprotoxic category 2” as from the 1st of December 2010. Formulations containing >0.3% of NMP have to be labelled as such. Consequently, the use of the solvent is restricted to professional users. NMP has been placed on the REACH “Substance of Very High Concern” (SVHC) list and is expected, sooner or later, to be placed under authorization or restriction.
Despite the efforts that have been directed to the development of substantially solvent-free aqueous polyurethane dispersions, as mentioned above, it remains an advantage to carry out the prepolymer mixing process in the presence of organic solvents or non-reactive diluents with the first aim to control the viscosity of the prepolymer. It is known that said organic solvent remains as a co-solvent in the dispersion thereby affecting the final properties of the aqueous polyurethane dispersions and articles, in particular coated articles made therefrom. Thus, the selection of the organic solvent is steered by the targeted properties of the aqueous polyurethane dispersions and articles made therefrom and by environmental legislation.
Therefore, there is still a need for environmentally friendly organic solvents especially suitable for use in polyurethane (PU) systems having an improved versatility, whereby said organic solvents can serve multiple purposes at the same time, in particular (1) acting as a prepolymer diluent in aqueous polyurethane dispersions (i.e. controlling the viscosity of the prepolymer), (2) acting as a dispersing agent (i.e. providing an improved stability of the PUD) and (3) acting as a coalescing agent thereby aiding in the formation of smooth, uniform films and coatings. There is thus an ongoing need for improved aqueous polyurethane dispersions comprising said organic solvents, present in minimal amounts which can provide products that are both chemically and colloidally stable, containing minimal amounts of internal and/or external water dispersible entities, that do not emit VOC, particularly PUDs having an improved ability to coalesce to films or coatings whereby said films and coatings obtained have improved mechanical properties, in particular tensile strength and elongation, better tear resistance and improved water resistance, and improved chemical resistance properties.